Communications signal amplifiers having independent power control and amplitude modulation

ABSTRACT

The present invention, generally speaking, provides methods and apparatus for producing an amplitude modulated communications signal, in which a constant-envelope carrier signal is modified in response to a power control signal to produce a modified constant-envelope carrier signal. The modified constant-envelope carrier signal is amplified in response to an amplitude modulation signal to produce a communications signal having amplitude modulation and having an average output power proportional to a signal level of the modified constant-envelope carrier signal. This manner of operation allows wide dynamic range of average output power to be achieved. Because amplitude modulation is applied after amplitude varying circuitry used to produce the modified constant-envelope carrier signal, the amplitude modulation is unaffected by possible non-linearities of such circuitry. In accordance with another aspect of the invention, operation in the foregoing manner at comparatively low average output power levels is combined with switch mode operation at comparatively high average output power levels, enabling high overall efficiency to be achieved. Hence, the disclosed modulator and amplifier combination, in addition to supporting very low power signals, also supports high power signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 09/834,024,filed on Apr. 11, 2001 and issued as now U.S. Pat. No. 7,010,276. Thisapplication is related to application Ser. No. 11/208,327, filed on thesame day as this application and also a continuation of application Ser.No. 09/834,024, filed on Apr. 11, 2001 and issued as now U.S. Pat. No.7,010,276.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communications signal amplifiers and topower control and amplitude modulation control.

2. State of the Art

High linearity and high energy efficiency are two principal (and usuallycompeting) objectives of conventional communications signal amplifierdesign. High linearity is required to produce “clean” communicationssignals that produce a minimum of interference and provide high qualityof service. High energy efficiency is desirable particularly forportable, battery-powered devices, as well as for purposes of reducinginfrastructure costs.

In a cellular telecommunications system, handsets and other devicescommunicating with the radio base station are required to transmit atone of many possible power levels depending on the proximity of aparticular device to the base station. This average output power controlfunction may for some standards entail a dynamic range of 80 dB.Achieving linearity over this wide dynamic range has proved problematic.

Referring to FIG. 1, a simplified diagram of a conventionalcommunications signal transmitter is shown. A signal generator 101generates a signal having both amplitude and phase modulation. Thissignal is applied to an amplitude varying circuit 103 such as a variablegain amplifier (VGA) or variable attenuator controlled in accordancewith a power level signal 104. A resulting scaled signal is then appliedto a linear amplifier 105 to achieve final amplification. In thisarrangement, because the amplitude-modulated signal passes through theamplitude varying circuit 103, the amplitude varying circuit must havevery linear performance.

An alternate approach involves polar modulation, in which separateamplitude and phase paths are provided. Polar modulation architectures(and similar architectures in which separate amplitude and phase pathsare provided) are described, for example, in U.S. Pat. Nos. 6,191,653,6,194,963, 6,078,628, 5,705,959, 6,101,224, 5,847,602, 6,043,707, and3,900,823, as well as French patent publication FR 2768574, all of whichare incorporated herein by reference.

Referring in particular to U.S. Pat. No. 6,191,653, a RF power amplifierarchitecture is described in which separate phase and amplitude pathsare provided. The amplifier has multiple stages. For signals having acomparatively high average power level, amplitude modulation is achievedin a final amplifier stage operating in a non-linear mode. In the caseof a FET (field effect transistor) active device, for example, drainmodulation is applied to the FET in order to impress the desiredamplitude modulation upon the output signal. For signals having a lowaverage power level, amplitude modulation is achieved in a precedingamplifier stage (again using drain modulation), the final amplifierstage being operated in linear mode (see column 6). In this manner, arange of amplitude modulation is provided that is larger than can besupported by just drain modulation of the final stage. Achieving an 80dB dynamic range of average output power using this arrangement,however, remains problematic. Furthermore, any non-linearities inamplitude modulation performed in the driver stage are magnified in thefinal stage.

The present invention addresses the need for wide dynamic range ofaverage output power without requiring high linearity. When applied inconjunction with switch mode amplification techniques, overall highefficiency may be achieved.

SUMMARY OF THE INVENTION

The present invention, generally speaking, provides methods andapparatus for producing an amplitude modulated communications signal, inwhich a constant-envelope carrier signal is modified in response to apower control signal to produce a modified constant-envelope carriersignal. The modified constant-envelope carrier signal is amplified inresponse to an amplitude modulation signal to produce a communicationssignal having amplitude modulation and having an average output powerproportional to a signal level of the modified constant-envelope carriersignal. This manner of operation allows wide dynamic range of averageoutput power to be achieved. Because amplitude modulation is appliedafter amplitude varying circuitry used to produce the modifiedconstant-envelope carrier signal, the amplitude modulation is unaffectedby possible non-linearities of such circuitry. In accordance withanother aspect of the invention, operation in the foregoing manner atcomparatively low average output power levels is combined with switchmode operation at comparatively high average output power levels,enabling high overall efficiency to be achieved. Hence, the disclosedmodulator and amplifier combination, in addition to supporting very lowpower signals, also supports high power signals.

BRIEF DESCRIPTION OF THE DRAWING

The present invention may be further understood from the followingdescription in conjunction with the appended drawing. In the drawing:

FIG. 1 is a simplified diagram of a conventional communications signalamplifier;

FIG. 2 is a simplified diagram of a communications signal amplifier inaccordance with the principles of the present invention;

FIG. 3 is a diagram of a communications signal amplifier like that ofFIG. 2 showing one particular amplification chain;

FIG. 4 is a block diagram of a communications transmitter in which thepresent invention may be used;

FIG. 5 is a more detailed diagram of the amplification chain of thepower amplifier of FIG. 4, in accordance with an exemplary embodiment;

FIG. 6 is a plot of device characteristics for the amplifier section ofFIG. 5;

FIG. 7 is a more detailed block diagram of portions of one embodiment ofthe communications transmitter of FIG. 4;

FIG. 8 is a block diagram of another embodiment of the communicationstransmitter;

FIG. 9 is a more generalized block diagram of the present communicationssignal transmitter;

FIG. 10 is a plot illustrating high power switch-mode operation;

FIG. 11 is a plot illustrating operation of a conventional amplifier;

FIG. 12 is a plot illustrating low power switch-mode operation;

FIG. 13 is a plot illustrating very low power, multiplicative-modeoperation;

FIG. 14 is a plot like that of FIG. 13, showing the boundary of a 5%linearity region;

FIG. 15 is a spectrum plot of an EDGE signal at 30 dBm; and

FIG. 16 is a spectrum plot of the same EDGE signal at −50 dBm.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, there is shown a simplified diagram of acommunications signal amplifier in accordance with the principles of thepresent invention. A signal generator 201 generates a signal typically(though not necessarily) having phase information. This signal isapplied to an amplitude varying circuit 203 such as a variable gainamplifier (VGA) or an attenuator, controlled in accordance with a powerlevel signal 204. A resulting scaled signal is then applied to anamplification chain 205. The amplification chain is responsive to acontrol signal 207 bearing amplitude modulation information (andpossibly power level information in addition to that of the power levelsignal 204) to produce an amplified communications signal (e.g., RFoutput signal) having the desired amplitude modulation and desired powerlevel.

FIG. 3 shows further details of the communications signal amplifier ofFIG. 2, particularly of the amplification chain. In the illustratedembodiment, the amplification chain is shown as having a single finalamplifier stage operating in multiplicative mode to achieve bothamplitude modulation and final amplification or, at low power levels,attenuation, as the case may be. The final amplifier stage includes athree-terminal active device Q (such as an FET) having a signal inputterminal IN, an output terminal OUT and a power supply input terminalPS. An amplitude modulation signal is applied to the power supply inputterminal. In this arrangement, because the amplitude-modulated signaldoes not pass through the amplitude varying circuit 303, the amplitudevarying circuit may have non-linear performance. To the extent thenon-linear characteristics of the amplitude varying circuit 303 may beascertained, they may be taken compensated for in setting the powerlevel signal 304 applied to the amplitude varying circuit.

Optionally, power level control, instead of being applied to the finalamplifier stage solely through the signal input path IN, may be appliedin combination through both the signal input path and the power supplypath PS.

The amplitude varying circuit 303 may be a variable gain amplifier (VGA)or a variable attenuator, controlled in accordance with a power levelsignal. The VGA or variable attenuator may be realized in one stage orseveral stages, using passive devices, active devices, or somecombination of the same, where the active devices (if any) are operatedin either “triode mode” or switch mode.

Referring to FIG. 4, a block diagram is shown of a communicationstransmitter in which the present invention may be used. A data signal401 is applied to a control block 403, such as a DSP, ASIC, ASSP, etc.In the general case, the control block produces a number of controlsignals, one or more of which are applied to each of a DDS/phase controlblock 405 (where DDS stands for direct digital synthesis) and anamplitude/power control block 407. In a specific embodiment, asdescribed below, the control block 403 produces a single phase controlsignal and multiple amplitude control signals.

An output signal of the DDS/phase control block 405 is applied to theamplitude/power control block 407. Responsive to the output signal ofthe DDS/phase control block 405 and to control signal(s) from thecontrol block 403, the amplitude/power control block 407 produces asignal output and a power supply output that are applied to a signalinput 409 and a power supply input 411, respectively, of anamplification chain 413 (shown in simplified form). The amplificationchain 413 has at least one amplification stage 413 a and typically twoor more amplification stages, as indicated in dashed lines, andamplifies the signal input 409 to produce an amplified communicationssignals such as an RF output signal, RF OUT.

The amplification chain of FIG. 4 may, in some embodiments, be operatedexclusively in “triode mode” or “multiplicative mode” (described morefully below), as opposed to switch mode, across the entire output signaldynamic range. In triode mode, with the active device operating in thetriode region, average output power increases with increase of thesignal input level and with increase of the power supply input level.Hence, for maximum average output power, levels of the signal inputlevel and the power supply input would be set to maximum levels subjectto the requirement of operating the amplifier in multiplicative mode.Similarly, for minimum average output power, levels of the signal inputlevel and the power supply input would be set to minimum levels subjectto the requirement of operating the amplifier in multiplicative mode.

When operated in this manner, power efficiency, while not especiallyhigh, is improved as compared to more conventional amplifiers in whichthe power supply input to the amplifier is held fixed. However,amplifier performance is somewhat sensitive to manufacturing andenvironmental influences. Increased power efficiency and dramaticallydecreased sensitivity to manufacturing and environmental influences maybe achieved by operating the amplification chain, during at least partof the time, in switch mode.

Referring to FIG. 5, a more detailed diagram of the amplification chainof the power amplifier of FIG. 4 is shown, in accordance with anexemplary embodiment. The amplification chain is shown as having threestages, each stage including an active power device. The active devicesare illustrated here as FETs (Field Effect Transistors) Q1, Q2 and Q3,but can be any three-terminal gain device. In the illustratedembodiment, the drain terminals of the FETs are each connected through arespective inductive choke, L1, L2 and L3, to a respective power supplyinput, V_(DD1), V_(DD2), V_(DD3). Hence, the power supply inputs of thethree devices may be controlled independently. The source terminals ofthe active device are coupled to a reference potential, e.g., ground.

A signal input is applied to a gate terminal of the first active deviceQ1, which produces an output signal at the drain terminal thereof. Thisoutput signal is applied to a coupling network CN1 that produces asignal input for the second active device, Q2, which is applied to thegate terminal thereof. Similarly, the output signal produced at thedrain of the second active device Q2 is applied to a coupling networkCN2 that produces a signal input for the third active device Q3. Notethat a parasitic capacitance, C_(GD), exists between the gate and drainof each of the active devices. This capacitance gives rise to a signalleakage phenomenon that has heretofore made the achievement of very widedynamic range on the output signal of the amplification chainproblematic.

The output signal produced by the third active device Q3 is AC-coupledthrough a capacitor to a load network. An RF output signal is developedacross the load network and may be transmitted, for example by anantenna.

In operation of the amplification chain of FIG. 5, drain modulation isperformed by applying an amplitude modulation signal to one or more ofthe drain supplies (V_(DD1), V_(DD2), V_(DD3)) while driving the signalinput at a high level to drive at least the final amplifier stage, andpreferably all the amplifier stages, into deep compression or switchmode. When the final amplifier stage is overdriven in this manner, thepower of the RF output signal developed across the load network isproportional to the square of the supply voltage on the terminalV_(DD3).

A normal mode of operation (as described, for example, in the above U.S.Pat. No. 6,191,653 in relation to a first embodiment thereof) would beto modulate the power supply input of the final stage (i.e., V_(DD3))and to operate the power supply inputs of preceding stages (i.e.,V_(DD1), V_(DD2)) at a fixed voltage. In this mode of operation,modulation of the power supply input V_(DD3) may require a voltage swingratio of 8:1 for signals such as that used in the EDGE system.

Where it is required that the RF output power be varied over a widerange of power levels (e.g., 80 dB dynamic range), some of this dynamicrange may be achieved by scaling down the applied amplitude modulationsignal, i.e., by increasing its dynamic range. This method, however, islimited by feed-through of the Q3 input signal to the output via thecapacitor C_(GD3). Leakage of the input signal to the output signalcauses AM to AM and AM to PM non-linearities at low levels of the powersupply input V_(DD3).

As described in U.S. Pat. No. 6,734,724 entitled POWER CONTROL ANDMODULATION OF SWITCH MODE POWER AMPLIFIERS WITH ONE OR MORE STAGES,issued on May 11, 2004, and incorporated herein by reference, theforegoing problem may be alleviated by reducing the power supply inputlevels V_(DD2) and/or V_(DD1), to minimize feed-through due to thecapacitor(s) C_(GD). This technique has been shown to achieve an outputpower dynamic range of 40 dB or more, considerably short of the 80 dBdesired for some cellular applications. A different approach is neededto achieve this extended dynamic range. Such an approach may be based onthe following observations.

When operating at low power levels corresponding to supply voltages ofless than 1V (e.g., a few or several tenths of a volt), the amplifierstages may no longer operate in non-linear mode, if the RF input driveto a stage is reduced sufficiently so that it is no longer of sufficientmagnitude to overcome the DC bias of that stage to turn it OFF. (Forexample, if the DC bias were set to −2V, and the FET threshold were−2.5V, any RF signal with magnitude 0.5V or less would not achieve the−2.5V necessary to turn the FET OFF.) FIG. 6 illustrates thecharacteristics of a typical active device, as well as a modulationapproach enabling extended dynamic range to be achieved. (The curvesshown in FIG. 6 are for a GaAs Metal Semiconductor Field EffectTransistor (MESFET) but would be similar for any FET device.) Themodulation approach indicated may be applied at a single stage of theamplification chain, such as the final stage. More preferably, drainmodulation is performed at the final stage, and drive modulation isperformed at the initial stage, as described presently.

Referring again to FIG. 3, in the case of the final stage, the operatingpoint is set by the power supply input V_(DD3) and by the gate voltageV_(G) applied from the previous stage. For a given V_(G) (i.e., givendrive voltage), as the power supply input V_(DD3) is changed, theoperating point of the of the device moves up and down the V_(G) curve,and the output amplitude is modulated accordingly, as a function of thechanging drain current IDD. This kind of modulation is indicated in FIG.6 as “Drain Modulation.”

To achieve the desired wide dynamic range, another kind of modulation isalso required, indicated in FIG. 6 as “Drive Modulation.” In particular,with the stage operating in the triode region, the output amplitude canalso be changed by changing the input drive to the stage. With allstages operating with low supply voltage, all stages are in triodeoperation, and the output power level can be further reduced (beyond alevel achievable with drain modulation alone) by reducing the inputdrive to the initial amplifier stage. Drain modulation is still appliedto the final stage through the power supply input V_(DD3), and at thesame scaling. This manner of operation allows the output power to befurther reduced, to a level of 80 dB or more down from the maximumpower, thus achieving the desired dynamic range.

Referring to FIG. 7, a more detailed block diagram is shown of oneembodiment of the communications transmitter of FIG. 4. Theamplification chain 713 includes three amplifier stages as previouslydescribed. A DDS/phase control block 705 includes a phase modulator/VCO706. An amplitude control block 707 includes circuitry for achievingboth drain modulation and drive modulation in the manner of FIG. 6. Inparticular, in the illustrated embodiment, three separate power controlsignals are provided by the control block 103 (not shown). One of thesesignals, Power Control 2, is applied in common to the power supplyinputs of the stages preceding the final stage. Another one of thesesignals, Power Control 3, is combined with an amplitude modulationsignal 711 from the control block, using a multiplier 708, to produce apower supply input 715 for the final stage. Using this arrangement,because the amplitude modulation signal 715 is independent of powercontrol signals, non-linearity in the power control signals does notaffect modulation linearity. Finally, a signal Power Control 1 sets thepower of the signal 709 into the amplifier by, in this embodiment,controlling the attenuation of a variable attenuator 710.

In operation, when a comparatively high average output power is desired,the signals Power Control 2 and Power Control 3 are set to a range wellbeyond that illustrated in FIG. 6 (e.g., more than about 3 volts), andthe signal input is minimally attenuated or subjected to gain, with theresult that the amplifier stages are all operated in deep compression,or switch mode. In this region, the signal input level has a negligibleeffect on output power. For purposes of output power control, the signalPower Control 1 could be set to zero attenuation. However, it isdesirable to provide substantial reverse isolation between theamplification chain and the phase modulator/VCO (e.g., to avoid VCOpulling). If the attenuator were set to zero attenuation, no reverseisolation would be achieved. By instead setting the signal Power Control1 so as to maintain at least some minimum attenuation, reverse isolationmay be achieved. The minimum attenuation is not so great as to preventswitch mode operation from being achieved. Amplitude modulation isapplied solely through the amplitude modulation signal.

When a comparatively low average output power is desired, the signalsPower Control 2 and Power Control 3 are set to within the rangeillustrated in FIG. 6 (e.g., a few or several tenths of a volt), withthe result that, with the gate bias values shown, the amplifier stagesare all operated in triode mode. In this region the signal input levelhas a substantial effect on output power. The signal Power Control 1 isused to apply drive modulation after the manner of FIG. 6. This effectsan additional control on average output power, independent of anyamplitude modulation, which is still applied through the amplitudemodulation signal.

For very low power signals, the drive signal will be stronglyattenuated, the degree of attenuation being reduced as desired signalpower increases, until some minimum attenuation level is encountered. Asdesired signal power further increases, the signals Power Control 2 andPower Control 3 are increased so as to resume switch mode operation.

Referring to FIG. 8, another embodiment of the communicationstransmitter is shown, similar to that of FIG. 7. In this embodiment, anamplification chain 800 includes multiple cascaded VGA stages (803, 805,807) followed by a final switch-mode power amplifier stage (SMPA) 809that, except for very low output powers, remains in switch mode. Amodulation generator 811 produces power control signals for each stage,including the VGA stages, which may be operated to have either positivegain or negative gain (i.e., attenuation), and the SMPA stage. Amultiplier 813 is provided that receives an envelope signal and a powercontrol signal for the SMPA stage from the modulation generator, formsthe product, and applies the resulting signal as the power supply to theSMPA. The power control signals for the VGA stages, in particular, areused to control the output power at very low power levels. For switchmode power control at moderate and high power levels, all of the powercontrol signals play an active role with the exception of the powercontrol signal for the initial stage, used more particularly for verylow power control.

A PM signal generator 815 receives modulation information from themodulation generator and a carrier signal and generates phase modulatedoutput signal, which is applied to an RF input of the amplificationchain.

The technical basis of the present invention may be further appreciatedfrom the following description with reference to FIG. 10 through FIG.16.

The approach followed, as previously described, entails switch-modeoperation at high and medium output power, and conversion to“multiplicative mode” at low output powers.

Referring to FIG. 10, the characteristic curves of a MESFET transistorare shown. (Other FET devices exhibit similar characteristics.) Beyond acertain drive voltage, the transistor is saturated and sources a maximumamount of current, I_(DSS), for existing load conditions. At relativelylow values of VDS, the current I_(DSS) is proportional to VDS inaccordance with a proportionality constant R_(DS), on. At increasinglyhigher values of VDS, the current I_(DSS) drops off by an increasingamount.

The operating space may be separated into two regions, a triode regionin which a change in drive voltage produces a proportional change incurrent, and a “saturation region” in which this proportionalrelationship no longer holds.

During high power operation, the switching transistor is always drivenhard, in switch mode. Hence, the transistor “snaps” between twooperating points, an ON operating point at I_(DSS), near R_(DS, on), andan OFF operating point beyond the threshold voltage V_(T), causing thetransistor to enter cutoff. A line joining these two points is analogousto a conventional load line; note, however, that at high power, thetransistor is not operated anywhere except at the endpoints for anyappreciable portion of time. Through the mechanism of drain modulation,previously described, the voltage VDS may be controlled. As a result,the load line may be shifted, with the transistor still being operatedin switch mode.

This manner of operation may be contrasted with that of conventionalamplifiers. In conventional amplifiers, as illustrated in FIG. 11, thetransistor [[is]] operates along a load line about a bias point definedby a bias curve. With no applied drive signal, the amplifier operates atthe intersection of the load line and the bias curve. Positive drivecauses the operating point to move up the load line, and negative drivecauses the operating point to move down along the load line. There is nosnapping between two operating points as in FIG. 10. Efficiency may beimproved using power-tracking methods in which the power supply voltageis reduced as the average output power is reduced. However, if the powersupply voltage is reduced to a degree that the device enters the trioderegion (as shown), potentially serious signal distortion results.

FIG. 12 and FIG. 13 show magnified views of curves of FIG. 10, closer intoward the origin. Referring to FIG. 12, at moderately low power, asimilar manner of operation as in FIG. 10 is followed. To reducefeedthrough effects, the drive signal is reduced, preferably to a leveljust sufficient to maintain switch mode. However, feedthrough effectsstill set a lower limit on the achievable average output power.Achieving average output power below this lower limit requires adifferent operating mechanism.

Such an operating mechanism is illustrated in FIG. 13. By reducing theinput drive to stay above cutoff, the operating point that in priorfigures corresponded to cutoff is now pushed in along the load line,into the “deep-triode” region. Operation of the device is no longerconfined to the endpoints. Rather, the operating point of the device ismoved along the load line by modulation of the drive signal (i.e.,modulation of VGS). Furthermore, the load line may be shifted toward oraway from the origin using drain modulation (i.e., changing VDS). Thismode of operation is referred to herein as “multiplicative mode,” sincethe device output is proportional to the product of the drive voltageand the drain voltage. Changing VDS effects amplitude modulation since,as the load line is shifted toward or away from the origin, the curvesgrow farther apart or closer together. Changing VGS effects powercontrol by increasing or decreasing the drain current ID.

By changing the bias level, the device may be caused to operate across adifferent subset of curves either closer to or farther away fromR_(DS, on). The multiplicative action of the device exhibits greaterlinearity and lower gain when the device is operated closer toR_(DS, on). FIG. 14, for example, shows a 5% linearity region withinwhich the multiplicative action of the device exhibits linearity within5%. The current through the device, ID, is only weakly affected bychanges in bias, implying good temperature stability characteristics.

The multiplicative nature of device operation in the deep-triode regionfollows from well-established device models. While these models chosenare specific to a certain kind of devices, it should be understood thatmany different kinds of devices exhibit similar behavior and may be usedin connection with the present invention. Triode tube devices could beused, for example, which have only a triode region and no saturationregion. Hence, the following mathematical description is exemplary only.

For FETs, device operation in the saturation region and in the trioderegion, respectively, is described by the following equations:I _(D) =K(V _(GS) −V _(T))²;I _(D) =K(2(V _(GS) −V _(T))V _(DS) −V ² _(DS)),where

$K = \frac{I_{DSS}}{V_{T}^{2}}$

The boundary between the saturation and triode regions is defined by thesimultaneous equationsV _(DS) =V _(GS) −V _(T)I_(D)=KV² _(DS)In the deep-triode region V_(DS)<<V_(GS)−V_(T), resulting in

$\begin{matrix}{I_{D} = {2{K\left( {V_{GS} - V_{T}} \right)}V_{DS}}} \\{= {2{K\left( {{V_{GS}V_{DS}} - {V_{T}V_{DS}}} \right)}}}\end{matrix}$Hence,I _(D)(t)=2K(V _(GS)(t)V _(DS)(t)−V _(T) V _(DS)(t))

The last term of the expansion may be considered a bias term V.sub.b(t);i.e., it varies at the envelope frequency (typically much less than 10MHz, as compared to the RF frequency of 1-2 GHz). By taking 2KV_(DS)(t)=G(t), the latter equation may be rewritten as:I _(D) =G(t)V _(GS)(t)+V _(b)(t)

The bias term is readily filtered out, using an output couplingcapacitor, for example, leaving the product of the envelope modulationsignal G(t) and the drive modulation (power control) signal V_(GS)(t).

Various methods may be used to control the transition between triodemode and switch mode in a smooth, glitch-free manner.

In one embodiment, a particular average power threshold, say −5 dBm, isidentified (e.g., empirically determined). For average power levelsabove this threshold, the amplifier is operated in normal“switched-mode”. For average power levels below this threshold, thevarious stages of the power amplifier are operated at conditionssuitable for achieving the threshold power output; reduction of theaverage power below the threshold level requires only reducing the RFsignal input level (e.g., by increasing the attenuation between thephase modulator and the first stage of the power amplifier). In thisway, there are clearly no glitches as the attenuation can be smoothlycontrolled; however, there is a potential for nonlinear power steps inthat a 5 dB increase in attenuation, for example, may not result in a 5dB decrease in average output power when operating close to thethreshold. Far below the threshold the output power versus attenuationcurve is very linear, but close to the threshold the output power versusattenuation curve is somewhat compressed. This compression needs to bepre-corrected for in the choice of attenuation applied to arrive at aparticular power level. Such adjustment is easily accomplished.

Another embodiment avoids the need for such adjustment, as follows. Whenoperating below the threshold, the various stages of the power amplifierare not operated at the threshold conditions but instead are operated atconditions appropriate for a higher power level (e.g., if the thresholdis −5 dBm, the power amplifier stages are operated as if the desiredpower level was, say 0 dBm). The attenuation before the first stage ofthe power amplifier must be greater this way, but the greaterattenuation substantially avoids the nonlinear relationship between anattenuation step and the resulting step in output power when outputpower is close to the threshold. Determination of the attenuationsetting to be used for the power level immediately below the thresholdmay be easily accomplished by one skilled in the art of RF measurements.Once this value is set, lower power levels may be achieved in linear dBsteps through linear dB steps of the attenuator.

Various other embodiments may be used to advantage. For example, in thelow output power mode in which all amplifier stages are operated intriode mode, instead of applying any amplitude modulation on the finalstage as described, amplitude modulation can be applied at any of thestages in the amplifier chain, including any of the stages preceding thefinal stage, using an appropriate amplitude modulator such as themultiplier previously described.

The desired amplitude modulation characteristic need not be appliedthrough a single signal or single stage. Rather, the desired amplitudemodulation characteristic may, if desired, be broken up into multipleamplitude modulation signals, the product of which yields the desiredamplitude modulation characteristic. These multiple amplitude modulationsignals may be applied to any combination of the amplifier stages andthe amplitude varying circuit. Furthermore, the foregoing modulationapproach may be applied to any stage so long as all succeeding stagesoperate linearly.

Furthermore, if desired, the same amplitude modulation signal may beapplied to multiple control ports. Applying the modulation to two portseffects a squared signal modulation, applying the modulation to threeports effects a cubic signal modulation, and so forth. When theamplifier is operated in this manner, each stage effectively acts as anindependent multiplier.

Referring to FIG. 9, a more generalized block diagram of acommunications signal amplifier is shown, embracing the foregoingvariations. A carrier signal C is applied to an amplitude varyingcircuit 903 responsive to a power control signal Pout.sub.1 to produce acarrier signal C′. The carrier signal C′ is then amplified by a linearamplifier chain 913 having one or multiple stages. The power supply ofat least one such stage is varied in accordance with a desired amplitudemodulation characteristic. The power supply input of each individualstage may be fixed (PS), varied in accordance only with a desired outputpower level (Pout) or in accordance only with a desired amplitudemodulation characteristic (AM), or varied in accordance with both adesired output power level and a desired amplitude modulationcharacteristic (Pout/AM). In some instances, power supply inputs ofmultiple stages may have identical AM components.

EXAMPLE

The foregoing method was applied to a demonstration board to control theaverage output power of a board-generated signal compliant with the EDGEspecification from a maximum output power of 30 dBm down to very lowoutput power of −50 dBm. The spectrum and error vector magnitude (EVM)of the output signal at both high power and at low power was measured.The results are shown in FIG. 15 and FIG. 16, respectively. Almost nospectral degradation or EVM degradation is observed.

Thus, there have been described power control methods and apparatus forachieving extremely high dynamic range power control with inherentaccuracy and outstanding signal fidelity over the entire dynamic range.To achieve low output power signals, the power supply inputs of theamplifier stages are set for triode operation, at very low (possiblynegative) gain. The power supply input of the final stage is amplitudemodulated (using drain modulation), and the amplitude of the drivesignal is adjusted in a series of steps through variable power supplyvoltage settings of the driver transistors and also through inputattenuation (drive modulation). As a result, it becomes possible toproduce output signals having average power anywhere within a widerange, or to greatly increase the dynamic range over which amplitudemodulation may be produced at a given average power level, or both.

With envelope modulation applied at the final amplifier stage, modulatorand amplifier noise are suppressed, with the result that the signalnoise floor is limited by the noise characteristics of the frequencydetermining element, i.e., the VCO. Independent control of envelopepower and average output power is made possible, which, although notrequired, allows the envelope insertion procedure to have fixed scaling,independent of output level. In any event, no variation of the outputnetwork (load line) is required. Furthermore, no triode distortionoccurs as in known power-tracking methods.

It will be appreciated by those of ordinary skill in the art that theinvention can be embodied in other specific forms without departing fromthe spirit or essential character thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalents thereof areintended to be embraced therein.

1. An apparatus for producing an amplitude modulated communicationssignal, comprising: an amplifier having at least one stage including athree-terminal active device having a signal input terminal, a signaloutput terminal, and a power supply input terminal, the three-terminaldevice being a non-switching stage; means for generating a carriersignal, the carrier signal being applied to the signal input terminal;means for generating a power supply input signal derived at least inpart from an amplitude modulation signal, the power supply input signalbeing applied to the power supply input terminal; wherein the at leastone stage produces the amplitude modulated communications signal inresponse to the carrier signal and the power supply input signal, asignal magnitude of the amplitude modulated communications signal at agiven instant being dependent on a signal magnitude of the carriersignal and to a signal magnitude of the power supply input signal. 2.The apparatus of claim 1, wherein the carrier signal is angle modulated.3. The apparatus of claim 2, wherein an average output power of theamplitude modulated communications signal is determined at least in partby a signal magnitude of the carrier signal, and amplitude modulation ofthe amplitude modulated communications signal is separately determinedby the amplitude modulation signal.
 4. The apparatus of claim 3, whereinthe power supply input signal is derived from both the amplitudemodulation signal and a power level control signal.
 5. The apparatus ofclaim 2, comprising an amplitude varying circuit responsive to a powerlevel control signal for controlling the signal magnitude of the carriersignal.
 6. The apparatus of claim 5, wherein the amplifier has multiplestages, including the at least one stage, following the amplitudevarying circuit, and wherein, during at least a portion of the time,each of the multiple stages is operated in triode mode.
 7. The apparatusof claim 6, comprising: another stage preceding the at least one stageand including a three-terminal active device having a signal inputterminal, a signal output terminal, and a power supply input terminal;and a power supply input signal for the other stage produced inaccordance with a desired average output power level and applied to thepower supply input terminal of the other stage.
 8. The apparatus ofclaim 5, wherein for at least one desired average output power level,the signal magnitude of the carrier signal and the signal magnitude ofthe power supply input signal are controlled such that the at least onestage is operated in switch mode.
 9. The apparatus of claim 1,comprising a load network coupled to a final amplifier stage is coupledto an output network, wherein a single configuration of the load networkis maintained across lowest power and highest power operation.
 10. Acommunications apparatus comprising: an amplitude varying circuitreceiving a constant-envelope carrier signal and producing a modifiedconstant-envelope carrier signal in response to a power control signal;and an amplification chain including at least one stage, theamplification chain receiving the modified constant-envelope carriersignal and an amplitude modulation signal and amplifying the modifiedconstant-envelope carrier signal to produce a communications signalhaving amplitude modulation and having an average output powerproportional to a signal level of the modified constant-envelope carriersignal.
 11. Circuitry for producing an amplitude modulatedcommunications signal, comprising: an amplifier having at least onestage, the stage including a three-terminal active device having asignal input terminal, a signal output terminal, and a power supplyinput terminal, a carrier signal generator, that generates a carriersignal; an amplitude-varying circuitry, coupled to the carrier signalgenerator, and coupled to the amplifier through signal input terminal,the amplitude-varying circuitry generating an amplitude-varied carriersignal; a power supply input signal generator, coupled to the powersupply input terminal, the power supply input signal derived at least inpart from an amplitude modulation signal, the power supply input signalbeing applied to the power supply input terminal; wherein the onenon-switching stage produces the amplitude modulated communicationssignal in response to the carrier signal and the power supply inputsignal, a signal magnitude of the amplitude modulated communicationssignal at a given instant being dependent on a signal magnitude of thecarrier signal and to a signal magnitude of the power supply inputsignal.
 12. Circuitry for producing an amplitude modulatedcommunications signal, comprising: an amplifier having at least onestage, the stage including a three-terminal active device having asignal input terminal, a signal output terminal, and a power supplyinput terminal, a carrier signal generator, that generates aconstant-envelope carrier signal; an amplitude-varying circuitry,coupled to the carrier signal generator, and coupled to the amplifierthrough signal input terminal, the amplitude-varying circuitrygenerating an amplitude-varied carrier signal; a power supply inputsignal generator, coupled to the power supply input terminal, the powersupply input signal derived at least in part from an amplitudemodulation signal, the power supply input signal being applied to thepower supply input terminal; wherein the one non-switching stageproduces the amplitude modulated communications signal in response tothe amplitude-varied carrier signal and the power supply input signal, asignal magnitude of the amplitude modulated communications signal at agiven instant being dependent on a signal magnitude of theamplitude-varied carrier signal and to a signal magnitude of the powersupply input signal.
 13. Circuitry for producing an amplitude modulatedcommunications signal, comprising: an amplifier having at least onestage, the stage including a three-terminal active device having asignal input terminal, a signal output terminal, and a power supplyinput terminal, a carrier signal generator, that generates a carriersignal that is angle modulated; an amplitude-varying circuitry, coupledto the carrier signal generator, and coupled to the amplifier throughsignal input terminal, the amplitude-varying circuitry generating anamplitude-varied carrier signal; a power supply input signal generator,coupled to the power supply input terminal, the power supply inputsignal derived at least in part from an amplitude modulation signal, thepower supply input signal being applied to the power supply inputterminal; wherein the one non-switching stage produces the amplitudemodulated communications signal in response to the carrier signal andthe power supply input signal, a signal magnitude of the amplitudemodulated communications signal at a given instant being dependent on asignal magnitude of the carrier signal and to a signal magnitude of thepower supply input signal.
 14. Circuitry for producing an amplitudemodulated communications signal, comprising: an amplifier having atleast one stage including a three-terminal active device having a signalinput terminal, a signal output terminal, and a power supply inputterminal; means for generating a carrier signal having a constantenvelope amplitude at a first magnitude, the carrier signal beingapplied to the signal input terminal; means for varying the amplitude ofthe carrier signal to a second magnitude; means for generating a powersupply input signal derived at least in part from an amplitudemodulation signal, the power supply input signal being applied to thepower supply input terminal; wherein the at least one stage produces theamplitude modulated communications signal in response to the carriersignal and the power supply input signal, a signal magnitude of theamplitude modulated communications signal at a given instant beingdependent on any change in the second magnitude of the carrier signaland to a signal magnitude of the power supply input signal.